Amorphous silicon and polycrystalline silicon thin films are widely used as semiconductor thin films in solar cells, sensors and so on. Polycrystalline silicon has a carrier mobility which is higher, by one to two orders of magnitude, than that of amorphous silicon. Further, polycrystalline silicon is thermally stable and therefore more reliable than amorphous silicon.
Accordingly, as a photovoltaic device having a polycrystalline silicon thin film, one is known in which single crystalline silicon or cast polycrystalline silicon is used as a substrate material and a polycrystalline silicon thin film is formed on this substrate by a liquid phase growth method, as disclosed in "Solution growth of poly-Si thin films for solar cells" in the Extended Abstracts (The 50th Autumn Meeting, 1989), Japan Society of Applied Physics, p.567.
When single crystalline silicon or polycrystalline silicon is used as a substrate for forming a polycrystalline silicon thin film by the liquid phase growth method as stated above, the cost of the device is extremely high.
Therefore, it would be more economical to employ ceramics, quartz glass, or the like as a substrate material; however, such a substrate has such an inferior adhesion characteristic with the polycrystalline silicon thin film to be grown in a liquid phase that the polycrystalline silicon thin film cannot be formed on the substrate by the liquid phase growth method.
The adhesion characteristic N (wettability) is the percentage of material which is attached to the surface of a substrate. N is defined by the following equation, wherein a value of N of 90% or below is considered as an inferior adhesion characteristic: EQU N=area to which material is attached/substrate area.times.100 (%).